Yuto MIYATAKE

We experimentally demonstrate compact, low-loss, and broadband 2×2 Si optical couplers on a Si photonic platform designed using the covariance matrix adaptation evolution strategy (CMA-ES). The measured minimum insertion losses (ILs) in the C-band were 0.071 dB, 0.016 dB, 0.016 dB, and 45%–55% optical bandwidth were 140 nm, 136 nm, and 139 nm for device lengths of 7.56 μm, 9.64 μm, and 12.32 μm, respectively. The ILs are smaller than ever reported in 2 × 2 optical couplers shorter than 10 μm, and the optical bandwidths are much larger than a conventional directional coupler. The results validate the effectiveness of the proposed CMA-ES-based design method, extending the scope of application of our design method to the design of passive devices on Si photonics platform.

We propose a low-insertion-loss (IL) and non-volatile optical intensity modulator based on Ge₂Sb₂Te₃S₂ (GSTS), a recently proposed Se-free wide-gap phase change material. Owing to the negligibly small optical absorption of amorphous GSTS at a wavelength of 1550 nm, a non-volatile intensity modulator with a low IL of less than 0.08 dB and a high extinction ratio (ER) of 19.6 dB is realized using GSTS with a device length of 8 μm, overcoming the trade-off relationship between IL and ER in conventional Ge₂Sb₂Te₅ (GST). Using visible laser light irradiation, we demonstrate multilevel modulation up to 24 levels by taking advantage of the small optical absorption of amorphous GSTS and the moderately large optical absorption of crystalline GSTS compared with GST. With the proposed GSTS intensity modulators, we demonstrate 4×4 matrix-vector multiplication using a recently proposed photonic tensor core, which shows a high tolerance to hardware error accumulation. On the basis of the experimental results, we numerically confirm that a high recognition accuracy of 97.33% can be obtained using a convolutional neural network with emulated optical convolutional operations.

An optical phase shifter based on phase-change materials (PCMs) is a promising building block of quantum photonic integrated circuits (QPICs) operating at mid-infrared (MIR) wavelengths on a Si photonics platform. In this article, we propose a record low-loss non-volatile PCM phase shifter operating at MIR wavelengths based on Ge₂Sb₂Te₃S₂ (GSTS), which is a new Se-free widegap PCM. On the basis of the refractive index and extinction coefficient spectra measured by spectroscopic ellipsometry, we show that GSTS has excellent material properties for optical phase shifters. By using GSTS in the MIR range, the optical absorption of a PCM phase shifter can be considerably reduced. We achieved an optical loss of 0.29 dB for a \pi phase shift, which is the lowest loss ever reported for a PCM phase shifter integrated with a Si waveguide. We also demonstrate the non-volatile resonance wavelength tuning of a microring resonator (MRR) with a GSTS phase shifter and the optically induced phase transition of a GSTS phase shifter based on laser irradiation.